The problem is to create an efficient flexible reflective film that is stable, corrosion resistant, and weather resistant. The reflective film must be flexible so that it can be used in a variety of configurations for solar energy applications, lighting reflectors, etc. To be efficient a reflective film must be highly specularly reflective to visible, ultraviolet, and/or near infra-red light between about 300-2,500 nanometers. It cannot be degraded by the high energy ultraviolet light emitted by the sun and conventional lamps. The reflective film must be dimensionally stable when exposed to weathering forces, so that the optical properties are preserved. Therefore, it must be impervious to moisture and have a low coefficient of hygroscopic expansion. The reflective film should be scratch resistant and resistant to the accumulation of dust and dirt on the surface. These characteristics would help preserve the optical properties of the reflective film under adverse conditions and lower the maintenance costs associated with cleaning the reflective surface. Preferably, the reflective film should also be low cost.
Attempts have been made to make such a film. For example, U.S. Pat. No. 4,307,150 describes the use of a painted layer of an interpolymer of certain acrylate and/or methacrylate copolymers to protect an opaque aluminum surface vapor deposited on a flexible polyester support sheet. The acrylate interpolymer paint is disclosed as about 5 g/m.sup.2 (0.3 mil) thick. The polyester support sheet consists of a biaxially oriented polyethylene terephthalate lamina containing conventional slip agents and therefore having a mildly irregular surface to facilitate winding and a second polyethylene terephthalate lamina containing no slip agents and therefore having an exposed surface which is essentially optically smooth.
The patent speculates that silver or other metals could be substituted for aluminum. It does not describe how this could be accomplished. Silver reflects visible light better than aluminum. The pure metals respectively reflect approximately 98% and 91% of the light striking their surface. When incorporated in structures of the type described in U.S. Pat. No. 4,307,150, the effective reflectivity is reduced by the presence of the protective coating, viz, to approximately 95% for silver and 85% for aluminum.
The 10% greater reflectivity of silver compared to aluminum in solar reflectors is significant, since the major cost of such reflectors resides in the hardware itself, rather than in the reflective metal foil that is incorporated into the hardware. In lighting fixtures the 10% difference in reflectivity is repeatedly multiplied as light from the lamp in the fixture reflects and rereflects off the surfaces. Therefore, utilizing a lighting reflector incorporating silver as opposed to one incorporating aluminum results in perhaps a 35-50% gain in effectiveness.
Although silver reflects visible light better than aluminum, incorporating silver in structures described in U.S. Pat. No. 4,307,150 presents major problems. For example, silver, compared to aluminum, is more susceptible to corrosion (including the well-known tarnishing). Pin holes present in an acrylate paint coating or openings along the peripheral portions of the metal-coated support sheet allow corrosive materials to reach the reflective surface. Furthermore, acrylate coating is hygroscopic and the invading moisture subjects the reflective metal to additional corrosion.
In addition, a thin layer of silver, unlike a thin layer of aluminum, is characterized by the presence of a spectral "window" through which ultraviolet light in the 300-400 nonometer region readily passes. The transmission of this light peaks at approximately 325 nanometers. Both the sun and conventional lamps emit ultraviolet light in this frequency range.
The ultraviolet light that passes through the silver causes the polyester support sheet behind the silver to degrade, releasing small bubbles of carbon dioxide gas. Since the gas cannot escape through the protective acrylate coating, it moves inward, causing bubbles in the adhesive that attaches the flexible reflective film to a rigid support. These bubbles impart a "chicken skin" appearance that reduces the optical and aesthetic qualities of the film. Roche et al., U.S. Pat. No. 4,645,714. Thus the structure described in U.S. Pat. No. 4,307,150 wherein silver is substituted for aluminum could not be used for an efficient weather resistant reflective film.
Incorporating corrosion inhibitors and/or UV absorbers in the protective acrylate coating does not solve the problems associated with using silver as a reflective material. Corrosion inhibitors ameliorate the corrosion problem, but they frequently impart an unacceptable color. UV absorbers placed in the acrylate coating to protect the polyester support sheet against degradation ameliorate the degradation of the polyester problem, but they frequently exacerbate the corrosion of the silver. Substituting other polymers for the polyester support sheet is possible, but biaxially oriented polyester is generally structurally superior. Many other polymers are also susceptible to UV degradation.
U.S. Pat. No. 4,645,714 proposes the use of corrosion inhibitors and UV absorbers in separate layers of acrylate interpolymer paint. The purpose of these separate layers is to keep the corrosive UV absorbers out of contact with the silver. The silver is first coated with a thin layer of acrylate interpolymer paint, about 1-4 g/m.sup.2 (0.05-0.2 mil), containing corrosion inhibitors. Then the product is coated with a second thin layer of acrylate interpolymer paint, about 4-8 g/m.sup.2 (0.2-0.4 mil) containing UV absorbers.
The first layer of acrylate interpolymer paint primarily functions as a vehicle for the corrosion inhibitor for the silver. The first layer can be thinner because its purpose is to place the dissolved corrosion inhibitors in contact with the surface of the silver. Corrosion inhibitor that does not bond with the silver atoms does not protect the silver and is wasted in a thicker layer.
The second layer of acrylate interpolymer paint is thicker, since its primary function is to serve as a physical barrier against the weathering elements. Its secondary purpose is to serve as a vehicle for the UV absorbers that should be kept away from the surface of the silver. A single UV absorber does not protect the polyester from the entire 300-400 nanometer range. The UV absorbers are effective to reduce the degradation of the polyester support sheet if placed between the ultraviolet light source and the polyester. The UV absorbers do not need to be adjacent to the silver.
The products described in U.S. Pat. Nos. 4,307,150 and 4,645,714 regardless of the type of reflective metal employed would not be suitable for many solar energy and lighting reflector applications. The thin acrylate paint coatings weather poorly and quickly erode away. Where the product incorporates a polyester support sheet and a layer of silver, the product suffers from the problems with UV degradation. As the acrylate layer containing UV absorbers is diminished, ultraviolet light degrades the polyester support sheet.
Even where the product is situated such that it is not exposed to weathering forces, acrylate paint does not adequately protect the metallized polymeric support sheet from moisture. Acrylate is stable to ultraviolet light, and so it has been used as an outer protective paint coating. But it is also hygroscopic and subjects the metal to additional corrosion.
When moisture invades the protective acrylate, the adhesives, and/or structural polyester film, the differing coefficients of hygroscopic expansion combined with the coefficients of thermal expansion for the various materials used in the film produce compressive stress. The expansion of the protective layers with respect to the metallized support sheet stresses the adhesive between the two. When the adhesive fails the reflective film buckles causing tunnels between the protective layer and the metallized support sheet. These tunnels allow wicking of moisture through the reflective film. The added moisture causes more damage.
Furthermore, the thin layers of acrylate paint are easily scratched, which reduces the optical and aesthetic properties of the film. A thicker protective coating of acrylate yields a more durable reflector, but the thicker the acrylate paint or cast acrylate layer, the more susceptible it is to cracking, buckling, and tunneling. Therefore, a product with a thick acrylate coating is not flexible enough to be incorporated into dish, trough, or angular configurations. Finally, a thick coating of acrylate paint is expensive.
Protecting the silver surface with a paint creates the possibility of pinholes in the paint. Pinholes allow spots of corrosion to develop, which diminishes the aesthetic properties and the optical properties of the reflective film. The thickness of a paint is also difficult to control, and some portions of the reflective surface receive even less protection.
Beyond these factors, dust and dirt easily adheres to surface coatings of acrylate paint, and so the reflective surface must be cleaned frequently.
All these factors reduce the optical efficiency an aesthetic properties of the reflective surface. To a lesser extent, with the possible exception of UV degradation of a polyester support sheet used with silver, the same problems are encountered when aluminum or other reflective metals are substituted for silver.